Chainsaws are traditionally made with guide bars, where the saw chain runs with the drivelink tangs in a groove along the perimeter of the guide bar and is carried at the front end of the guide bar by a toothed sprocket inserted in the guide bar. The guide bars are either solid, i.e., made from a single steel plate with a milled or ground groove, or laminated, i.e., made from three thinner plates joined by spot welding, with the groove being created by the middle plate being smaller than the side plates. The guide bars are usually hardened along the edges to improve the wear resistance where the chain slides along the guide bar, and unhardened or have a lower hardness between the edges.
It has been shown, however, that guide bars of the known types when used in vehicle-born and stationary tree harvester machines are easily damaged if, for example, the grip of the machine around the tree trunk is not firm enough. If the grip is not firm enough, the tree can slide downward onto the chain saw bar, thus causing a considerable bending force to be imparted. A common type of damage is when the guide bar is bent near the end attached to the machine where the bending moment is at a maximum. It is often difficult to straighten conventional guide bars without creating cracks in the hardened edge and fractures near the spot welds (in laminated guide bars), where hard brittle regions border on soft heat affected zones.
U.S. Pat. No. 5,052,109 discloses that soft annealing of a narrow zone across the width of the guide bar near the end attached to the machine will concentrate any bending to this bending zone, where the hardness both at the weld spots and at the previously hardened edges is low enough to avoid fracture to a degree, and that such a guide bar is easier to straighten after it has been bent. Also, U.S. Pat. No. 5,052,109 teaches spacing the narrow soft annealed zone to be adjacent to the mounting bracket. An apparent result of positioning of the soft annealed bending zone to be spaced in front of the mounting bracket is that the part of the bar covered by the mounting bracket is subjected to lower transverse forces, thereby reducing chances of damage thereto. Disadvantages with this method are that the force a guide bar can withstand without bending is lower, and that the edges may become wrinkled during the concentrated bending and it may be difficult to get the edges smooth when an attempt is made to restraighten the bar. In other words, to make it easier to straighten, the bar is made easier to bend. This has the result of the bar being bent more readily, which requires the bar to be removed from the harvester and restraightened more often. The restraightening process is labor intensive and results in considerable down time for the mechanical harvester. Also, because a restraightened bar has a considerably shorter life than a bar not restraightened (due to tempering inherent to the restraightening operation and the difficulty in getting the edge rails smooth), the net result of softening a zone in guide bars in accordance with the U.S. Pat. No. 5,052,109 is a relatively short guide bar life.
A solution to the problems associated with guide bars bending is proposed in co-pending U.S. patent application No. 08/239,316 filed on May 6, 1994, now U.S. Pat. No. 5,561,908 in the name of Arvo Leini. This solution involves making the bar more resistant to bending by providing a relatively hard body. The relatively hard body provides a spring-like action to bending forces. For instance, when subjected to a transverse force due to a tree slipping in a mechanical harvester gripper, the bar will deflect a considerable degree, and, when the force is removed, the bar will usually spring back to its original shape. The advantages are clear. Since the bar is not bent, it does not need to be removed from the harvester for restraightening. The bar's deflection, however, might otherwise result in the hardened edge rails cracking at the point of deflection even when the bar springs back to its original shape. The Leini invention includes the feature of interrupting the edge hardening of the guide bar rails in front of the mounting bracket to avoid this possibility. The bending is, however, never limited to the area outside of the mounting bracket. Due to the strain in the clamping bolts when the bar is deflected, the mounting bracket will open a small amount, and part of the bar within the mounting bracket will also be deflected as far back as the clamping bolts. This part of the bar is especially vulnerable due to small oil holes in proximity of the edges, acting to raise the local stress above that created in a part of the bar without holes. It is previously known to omit edge hardening adjacent to the oil holes of guidebars for hand-held chainsaws, but such guidebars differ from bars for harvesters mainly in size, hand-held chainsaws having bar thickness around 4 mm, harvesters bars having a bar thickness around 5-6 mm. Omitting the hardening adjacent to the oil holes also reduces the risk of cracking during manufacture, in the event thast the hardening depth is incorrectly adjusted, or in event that the oil holes are moved to be more closely adjacent to the edges and/or a thinner bar are used.
The present invention concerns a guide bar where the risk of cracks at the edges due to bending of the bar and due to the proximity of the oil holes to the edge rails is reduced or eliminated without lowering of the bending resistance, and where any bending will not be so concentrated that the edges could get wrinkled.